1. Field of the Invention
This invention relates to polymer electrolyte membrane fuel cells, polymer electrolyte membrane fuel cell stacks, and, in particular, low cost metal bipolar plates and current collectors for polymer electrolyte membrane fuel cells and fuel cell stacks.
2. Description of Prior Art
A polymer electrolyte membrane fuel cell is an electrochemical device capable of producing electricity (and chemical compounds), which is a promising option as a power source for electrical vehicles. Polymer electrolyte membrane fuel cells comprise an anode electrode, a cathode electrode and a polymer electrolyte membrane disposed therebetween. To provide sufficient electrical output, individual fuel cell units are stacked, one on top of the other, with electrically conductive bipolar plates disposed between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit.
Substantial drawbacks to widespread acceptance and implementation of polymer electrolyte membrane fuel cell systems as a power source for transportation applications are the high costs and the low power density of the fuel cell systems. A significant contributor to these drawbacks is the bipolar plate which electrically connects adjacent fuel cell units in the fuel cell stack and which provides the fuel and oxidant supply to the fuel cell units comprising the fuel cell stack. Currently, graphite is the most commonly used material for producing bipolar plates (See, for example, U.S. Pat. No. 5,942,347). On the positive side, graphite is resistant to corrosion, has low bulk resistivity and low specific density, and provides low contact resistance with the electrodes. On the negative side, graphite is high in cost, difficult to machine and brittle, the latter problem requiring that the bipolar plates have a thickness on the order of several millimeters, making the fuel cell stack both heavy and voluminous.
To be acceptable in the polymer electrolyte membrane fuel cell environment, a material for producing bipolar plates must provide high electronic conductivity, low corrosion, zero gas diffusivity, good chemical compatibility with other components of the fuel cell, low cost, low weight, low volume, sufficient strength, suitability for thermal control and suitability for fabrication. Metal plates have been proposed as an alternative to graphite plates. However, as a general rule, metal plates have a high corrosion rate in the polymer electrolyte membrane fuel cell environment, resulting in the migration of corrosion products (multivalent metal cations) to membrane sites and a reduction in proton transfer rate. In addition, metal plates are subject to the formation of insulating (high resistance) passivating layers. Noble metals are viable candidates but are very expensive. Stainless steel is a potential candidate in that it is low in cost, easy to shape and can be used in sheets as thin as 0.1 to 1.0 mm, thereby providing a low volume stack. However, stainless steel is chemically unstable in the fuel cell environment. It has a high corrosion current when in contact with the acidic electrolytic membrane in the operation region of the fuel cell and it cannot work if the pH is less than 5. To circumvent these problems, a protective coating may be applied to the stainless steel plate. However, this requires an extra processing step and adds both cost and weight to the plate and, thus, the fuel cell stack.
It is, therefore, one object of this invention to provide a low cost metal bipolar plate for polymer electrolyte membrane fuel cell stacks.
It is another object of this invention to provide a metal bipolar plate for polymer electrolyte membrane fuel cell stacks which is chemically stable in the polymer electrolyte membrane fuel cell environment and which does not require application of a protective coating.
It is another object of this invention to provide a metal bipolar plate for polymer electrolyte membrane fuel cell stacks which has good chemical compatibility with other components of the fuel cell stack.
It is still a further object of this invention to provide a metal bipolar plate for a polymer electrolyte membrane fuel cell stack which has high electronic conductivity. By xe2x80x9chigh electronic conductivity,xe2x80x9d we mean greater than about 7.5xc3x97103 s/cm.
These and other objects of this invention are addressed by a polymer electrolyte membrane fuel cell stack comprising a plurality of substantially planar fuel cell units, each of which comprises an anode electrode, a cathode electrode and a polymer electrolyte membrane disposed therebetween. A metal bipolar plate is disposed between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit. The bipolar plate comprises a chromium-nickel austenitic alloy, wherein the chromium and nickel, on a combined basis, comprises at least 50% by weight of the alloy. In accordance with a particularly preferred embodiment, the percentage by weight of nickel in the alloy is greater than the percentage of chromium.